There is increasing recognition that the unique nature of tumor cell metabolism, which is considered a hallmark of human cancer, is a promising research area for the discovery of new therapies. In this pilot (R21) project, we propose to investigate the potential of a key stress-inducible metabolic regulator?the orphan nuclear receptor estrogen-related receptor ? (ERR?)?as a target for disrupting the central carbon metabolism of aggressive (triple negative, TN) human breast tumors. Our proposed project emerged from the convergence of two lines of research on tumor metabolism: i) mechanistic studies in which we found that an experimental antitumor agent called SR16388 directly inhibits ERR? activity; and ii) basic studies in which we confirmed that the cellular energy sensor 5'-AMP-activated protein kinase (AMPK) can drive transcription of the gene for the major binding partner of ERR?, peroxisome proliferator-activated receptor ? co-activator 1?/? (PGC-1). Considering that the genes for ERR? and PGC-1 are both positively self-regulated, we reasoned that the ERR?/PGC-1 transcriptional complex would be uniquely sensitive to inhibition of either component. Here we will focus on ERR? itself as a potential therapeutic target in experimental tumors containing metabolically stressed microenvironments; specifically, we will investigate the hypothesis that ERR? is a novel metabolic target in aggressive [TN] human breast cancer (Specific Aims). To investigate this hypothesis, we will use TN breast cancer cells in knockdown studies to understand how ERR? supports the central carbon metabolism of experimental breast tumors and thus enables tumor growth; these studies will involve quantitative stable- isotope metabolic analyses of cell cultures and orthotopic tumors with 13C-labeled D-glucose or L-glutamine tracers (Aim 1). In the studies for Aim 2 we will perform pilot translational studies to determine whether targeting ERR? with established small-molecule inhibitors of ERR? (including our novel inhibitor, SR16388) will inhibit the growth of experimenta breast tumors. In summary, the studies for Aims 1 and 2 are designed to test the proof-of-concept that disruption of the ERR?-dependent carbon metabolism of TN breast tumors could be used clinically, likely in an adjuvant setting with established therapies.